GENERAL AVIATION Light aircraft design conference
Flying lightly into the future
CHRIS WRIGHT from the RAeS General Aviation Group reflects upon the GA Light Aircraft Design Conference and its focus on the future.
The UR-1 e-Racer.
This year’s annual General Aviation (GA) Design Conference followed closely on the heels of COP26. Although the theme of the overall design conference was not explicitly stated (an omission), it was encapsulated in the theme for the last session: ‘The Way Ahead for GA’. The concerns about climate change are bringing about an ever-sharper focus on lifestyle and leisure and, in particular, on the use of hydrocarbon fuels. The conference comprised a tightly packed programme, based on eleven talks, most of which related directly or indirectly to this theme.
Catamarans
The first session started with two topics closely allied to light aircraft – C Class catamarans designed for the Little America Cup races and a radical human-powered aircraft.
The elegant Invictus wingsail structure.
Yacht racing has seen a remarkable development in achievable top speeds, employing highly sophisticated flapped rigid wing structures and advanced hydrodynamics. Norman Wijker, currently CEO for SAMAD aircraft, previously ran the UK C Class team for several years and gave an account of yacht development. Of particular interest is the wingsail structure, seen above.
The wingsail is of high aspect ratio (over six) and comprises two parts: a symmetrical aerofoil leading section with a torsionally stiff cantilever mast buried within the elegant structure and a large chord symmetrical section flap along the whole length of the wingsail, supported by the trailing edge of the leading section. The flap leading edge is neatly supported on a travelling hinge which enables the flap to traverse to become a high lift blown flap on either side of the trailing edge, according to which tack the boat is on. The hinge arrangement is visible above.
The covering is plastic film and the total weight of the extremely robust wingsail is a remarkable 40kg for a length of approximately 13.5m. This directly relates to advanced light aircraft structures and aerodynamics. The elegant Invictus catamaran is seen above.
Flying wing
Hania Mohiuddin chairs the British Human Powered Flying Club. She described the Volaticus Human Powered Aircraft (HPA) design-and-make project. Hania and a team completed this project as part of their degree course in Aerospace Engineering at Sheffield University.
The wingsail cross-section, indicating the floating hinge arrangement
They opted for a radical flying wing design to minimise weight, achieving the remarkably low airframe weight of 21kg. She described the processes of choosing configuration, aerofoil selection and optimising the propellor design, which all contributed to a stimulating and satisfying understanding of whole aircraft design. The manufacturing activity was well illustrated and the resulting aircraft was tested at the 2014 Icarus Cup meeting – an annual event for HPA.
Testing confirmed the wing design and structure, and that more work was required on the fuselage and power system. This was progressed in 2015 and more testing took place in 2016. Sadly, the team has moved on and this promising project has not yet been followed through to its full potential.
Hania went on to describe other projects and how HPA technology has been employed in related projects – extremely low wing loading inflatable craft, high altitude long endurance (HALE) solar-powered surveillance aircraft and even the propeller design on the successful Mars Rover drone. The team members will have gained invaluable experience in lightweight structures, aerodynamic optimisation, manufacturing and testing which all relate to future design in light aircraft. The project was also a wonderful example of what can be achieved with the right setting and appropriate encouragement.
The Skyranger
The Skyranger is a microlight aircraft that has been steadily developed over the last two decades through to its present form: the Nynja.
The Invictus C Class catamaran for the Little America Cup.
Paul Dewhurst, Director of Flylight Airsports, outlined the development journey from the original French design in 1994 by Philippe Prévot through to the present design, which takes advantage of the rise in weight up to 600kg in the UK two-seat microlight category.
When initially introduced in this country a number of significant improvements were required for UK approval, including a stiffened and strengthened control system, nose leg reinforcement, a firewall and an increase of fin area.
Subsequent changes included re-engining and reduced wingspan – creating the Swift. Further changes refining the fuselage, wing section and tips led on to the Nynja.
Paul Lewis is a consulting airworthiness engineer who has been involved throughout the Skyranger project and many other certification projects for small aircraft types.
The recent change to the microlight regulations, permitting a maximum take-off weight increase from around 450kg up to around 600kg, has been applied to the Nynja, and Paul has been involved in addressing the design and certification changes required.
After detailing the technical development of the Skyranger series, Paul illustrated the redesign and testing – shown here – of the required more robust undercarriage legs and the greater load-carrying ability of the wings.
The previous GA Design Conferences in 2019 and 2020 focused on electrification. This is becoming evermore relevant. Drivers for electrification are growing steadily stronger with the push towards greener aviation and the pull of the continuing progress in electrical and electronic technology.
The GA sector is ideally positioned for electric aircraft development. The energy density of lithium batteries is sufficient to approach reasonable endurance for efficient airframes and to give high energy for short-duration activities, such as air racing, training, parachute dropping, glider tugging, glider self-launch, sustain and more.
The build process and Volaticus and team at the Icarus Cup 2016
Electrifying aircraft
The next talk directly addressed this focus – EnabEl: Enabling Electrification of Aircraft. This was given by a team of three from Cranfield heading up the EnabEl programme: Dr Guy Gratton, Associate Professor of Aviation and the Environment; Dr Bahareh Zaghari, lecturer in Propulsion Integration and Dr Tom Delaney, Research Fellow majoring on hybrid and all-electric aircraft propulsion systems. Guy explained that EnabEl is a consortium of three companies: the light aircraft company (TLAC), Flylight, and cdo2 – a battery monitoring and technology company and Cranfield University.
The consortium is working with close support from the BMAA, CAA and ARUP Design Consultancy, plus government funding from Innovate UK to encourage the building of electric light aircraft. Guy emphasised the climate emissions from aviation, which are estimated to be growing 3% per year and need to be curtailed. Light aviation is the best platform to develop technologies for future transport use and is in the best position to benefit from these technologies. The EnabEl plan is to explore the Design, Build, Fly and Certification of electric microlights with two companies: electrifying the TLAC Kub and making the Flylight Nynja hybrid capable.
 The latest 600kg Nynja. |
 The main undercarriage at maximum splay in its drop test – from which it fully recovered. |
The TLAC Sherwood eKub with batteries mounted in the wings.Alongside the aircraft projects are parallel activities to develop charging facilities and operational experience for airports and for training. Tom described the challenges in electrifying the compact single-seater Kub (G-EKUB): installing batteries into the wings, the powertrain installation and fitting instrumentation into a small cockpit panel.
Although the Kub is an SSDR, the EnabEl team is electing to apply Section S for safety and for developing the special conditions for electric aircraft working with the CAA.
Bahareh outlined the Nynja programme: following a fully electric conversion, hybridisation ground testing is planned. She then described a parallel noise measurement programme, which will be particularly important when applied to the burgeoning activity in air taxis and UAM vehicles.
Although quieter than helicopters, the environmental noise aspects of eVTOL vehicles are going to be monitored very closely as ‘into city’ air taxis become a reality.
Bahareh also outlined the possible follow-on programme: EnabEl-2 and a growing number of exciting electrification programmes at Cranfield with further developments in hybridisation, the TELUM a zero-emission training aircraft project, and much more.
The after-lunch session started with accounts of the RAeS GA group activity and initiatives, the first being an E-Conditions update.
John Edgley, past chair of the GA group and the initiator of Experimental Conditions, gave some of the background of the formulation of E-Conditions between the RAeS GA group and the CAA. E-Conditions, which are now enshrined in CAP 1220, are designed to make the path of developing new aircraft and aircraft modification easier.
The success of E-Conditions was well illustrated by the fact that ten projects have been registered in the last five years and most of these have progressed to the flying stage.
A notable E-Condition programme has been the ACCEL Spirit of Innovation aircraft constructed by Rolls-Royce and ElectroFlight which, since the conference, has captured the electrically-powered aircraft world speed record.
Further projects are ‘in the wings’, notably by two very active competent persons – Alex Ellin and Bill Brooks.
Overall, E-Conditions has proved very successful and it is hoped that this initiative, together with the Design, Make, Fly initiative and the outcomes of the design competitions, will combine to create exciting future developments and, hopefully, refreshed and regenerated general aviation manufacturing.
Design Make Fly Initiative
Led by Mike Jump, a Senior Lecturer in Aerospace Engineering at the University of Liverpool, a GA working group has been looking at the opportunity that exists to revolutionise small aircraft design, manufacture and operation to meet potentially large new markets.
The working group has been considering the problems and barriers, as well as consulting with interested parties and successful teams (such as Akafliegs and Delft University), to formulate proposals.
The proposals draw together the potential available in the academic and manufacturing environments working together, the stimulus of the Annual Design Competition and the opportunity presented by the E-Conditions initiative. It is intended that the proposals will be published mid-2022 and, in a similar manner to E-conditions, will spur enthusiasm and growth.
Clockwise from top left: The all-electric Pipistrel Alpha Electro, a 1911 Curtis Waterbird, the FlytCycle and the ACCEL Spirit of Innovation.
The general aviation light aircraft annual design competition
For a number of years Tony Bishop has led the very successful annual Light Aircraft Design Competition. Tim Watkins, a Flight Test Engineering Instructor at the ETPS, Boscombe Down, has now taken on this mantle.
Cranfield UniversityIt has been decided that it should synchronise the design competition with the academic year in order to enable academic and school teams to take part. In consequence, the design competition and entry details will be announced each May with winners being presented at the design conference in each subsequent year. Accordingly, for the competition announced in May this year the winners will present their submissions in November 2022.
Tim started by summarising some of the previous winning submissions, one of whom, Orca Aerospace, is gathering partners and potential customers, and is working on funding for the development of its eVTOL for Medical Evacuation submission. The 2021/2022 competition is for a 600kg STOL microlight aircraft with electric propulsion, for a pilot plus a passenger or cargo, able to operate from austere and remote airstrips in developing countries. The aircraft should be suitable for recharging via solar farms, batteries and chargers at each airstrip.
More details are on the RAeS website: https://www.aerosociety.com/news/raes-internationalgeneral-aviation-design-competition-20212022.
Illustrating the General Aviation Design Competition 2021/2.
Brian Stirm of Purdue University is a technical expert on aircraft design and fabrication as applied to light aircraft and flight vehicles.
The title of his talk was ‘Able Flight: adapting Light Aircraft for the Disabled’. The Able Flight mission is to assist pilots to train toward gaining the Light Sport Pilot certificate.
The US Light Sport Aircraft category is equivalent to the 600kg microlight category in the UK.
Brian started his talk with consideration of what constitutes ‘disabled’.
Brian presented the challenge that flight control systems constitute a major part of an aircraft and that any modifications may require special attention to detail and with possible customising and subsequent extensive flight proving.
Some manufacturers have full-hand control systems that can be fitted at the outset and he cited the Sky Arrow, which has a left-hand stick that enables rudder control (fore/aft) and throttle control with a twistgrip.
Brian left us with some thoughts that in some cases apply to all pilots – some aircraft apparently being designed only for fit young pilots!
Design Considerations for Disability.
The Way Ahead for GA
In the first talk of the afternoon session: ‘The Way Ahead for GA’, Tim Watkins gave a clear technical presentation on ‘Optimising the Range of Small Aircraft’. A survey he conducted on a sample of 28 current propeller-driven GA aircraft indicated that, by and large, fresh designs are required rather than re-engining with electric power systems. Modifying the Breguet range equation for electric aircraft, he considered the aspects of powertrain efficiency, battery-specific energy, battery mass fraction (of aircraft take-off mass) and lift-to-drag ratio. Very reasonable optimisation of all these parameters in fresh designs was shown to lead to a range multiplication of 2.7 over typical existing designs. Tim also mentioned the benefit of using a high aspect ratio wing for these applications and all the designs shown appear to have adopted this.
Examples of new designs.
Sergey Kiselev from ZeroAvia gave a presentation on zero-emission aviation: ‘Making Hydrogen Aviation Possible’. Sergey proposed hydrogen as the only scalable zero-emission solution, applicable up to the largest transport aircraft. He compared fuel cell-driven solutions with jet engines indicating that: hydrogen fuel costs are on track to undercut jet fuel, fuel cells are beginning to show extremely long maintenance life, noise levels are lower without jet noise and distributed thrust systems can give much higher efficiency.
The figure on the facing page shows a progressive approach for adoption of hydrogen hybrid power, starting with commuter aircraft and with applications developing to suit the largest transport aircraft. ZeroAvia has been testing the powertrain systems on a six-seater Piper aircraft and is developing systems for 10-20 seat and 40-80 seat aircraft. Although not focusing its efforts on general aviation, there is every chance that GA will benefit in the future.
Re-powering GA
Tony Bishop, member of the RAeS GA committee, reviewed why GA is having to change in the context of the international swing away from hydrocarbon fuel in order to combat climate change. Tony outlined the benefit of disruptive technologies and the rapid rise of renewable energy. Renewable energy is providing a practical solution to, initially, short-range and short-duration GA aircraft. The technologies of improved battery energy density – pushing towards 500KWh/Kg through chemistry and packing, improved electric motors and control systems, refined aerodynamics and structures all point to what is achievable, short-term and by 2040, summarised in the figures above. The opportunities are great and the continuance of GA appears assured.
It had been hoped that a panel would engage in discussion of the Way Ahead for GA but, unfortunately, a streaming failure interrupted the conference for about 40 minutes, leading to the cancellation of the panel discussion item. Perhaps, in the near future, this panel may be convened with a more generous time for discussion and be able to grapple with these issues more comprehensively from varying standpoints. The topic is not going to go away.
 What is possible by 2025 and 2040. |
What is possible by 2025 and 2040.
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Racing batteries
Marc Umbricht, CEO of Pie Aeronefs gave the final talk of the conference: ‘Air Race E: the Pie Aeronefs UR-1’. He described the layout of the UR-1 and, in particular, the design of the wing-mounted battery system. Air Race E has attracted much attention and several projects are based on the re-engining of the Cassutt racer. Marc Umbricht’s team is breaking out of this mould and has produced a very attractive design – the UR-1. It has an elegant slimline profile and notably, a V tail. Marc detailed the wing-mounted battery layout, which is a design feature.
NASA’s X-57 Maxwell. NASA
Batteries mounted in the wing reduce the wing-bending moments, which in a race context have to withstand 8g and landing shock is reduced by wing mounting the main undercarriage, which in this case is retractable. Wing-mounted batteries enable effective cooling and concentrate much of the aircraft mass near the cg.
The heavy batteries and high currents are kept well away from the cockpit – a safety advantage. Difficulties with this layout include battery access and maintenance, wiring and, to an extent, the effect of higher polar inertias on structure and handling. We look forward to the emergence of the flight vehicle and its performance in Air Race E. In his introduction, Marc also mentioned the Pie Aeronefs design for a four-seater aircraft with a distributed power system.
In conclusion, 2021 was an exciting year for electrification. As mentioned, the ACCEL Spirit of Innovation, described in our two previous conferences, has now captured the World Air Speed Record for electric aircraft. We congratulate the ACCEL team of Rolls-Royce and Electroflight on their success.
Earlier in the year the RAeS GA group hosted a lecture by Vertical Aerospace on its eVTOL projects, and, in particular, their new air taxi design the VA-X4.
This project has clearly hit the right notes in terms of capability, performance and ‘looking right’ – as agreed by many customers. At the time of the conference the project had considerably more orders and provisional orders than the nearest competitor.
Again, we congratulate Vertical Aerospace on its progress and wish them the very best as they forge ahead with their VA-X4 programme.
We thank all the speakers at the Design Conference for their fascinating and stimulating contributions and we thank all the attendees who linked in. We are also very grateful to the IMechE for its sponsorship which helped to make our conference possible.
We welcome all proposals for GA Design and Technology subject areas to explore for the next conference, and we look forward to you joining us in 2022.